C++ Is private really private?

Question

I was trying out the validity of private access specifier in C++. Here goes:

Interface:

// class_A.h

class A
{
public:
    void printX();
private:
    void actualPrintX();
    int x;
};

Implementation:

// class_A.cpp
void A::printX()
{
    actualPrintX();
}

void A::actualPrintX()
{
    std::cout << x:
}

I built this in to a static library (.a/.lib). We now have a class_A.h and classA.a (or classA.lib) pair. I edited class_A.h and removed the private: from it.

Now in another classTester.cpp:

#include "class_A.h"    // the newly edited header

int main()
{
    A a;

    a.x = 12;           // both G++ and VC++ allowed this!
    a.printX();         // allowed, as expected
    a.actualPrintX();   // allowed by G++, VC++ gave a unresolved linker error

    return 0;
}

I know that after tampering a library's header all bets are off (I mean, system integrity, etc.) Albeit the method being hacky, is this really allowed? Is there a way to block this? Or am I doing something wrong here?

Answer 1

private is not a security mechanism. It's a way of communicating intents and hiding information that other parts of your program do not need to know about, thus reducing overall complexity.

Having two different header files is not standards compliant, so technically you're entering undefined behaviour territory, but practically, as you've found, most compilers won't care.

Answer 2

You've strayed beyond what's allowed in C++, so what you're doing isn't allowed - but of course it may work on some compilers in some situations.

Specifically, you're violating the One Definition Rule.

This article by Herb Sutter explains it quite nicely - it also provides a legal and portable way of circumventing the access specifier system.